Microporous thermoplastic resin membranes are widely used in various applications such as battery separators, electrolytic capacitor separators, various filters, moisture-permeable, waterproof clothes, reverse osmosis membranes, ultrafiltration membranes, microfiltration membranes.
The production of microporous thermoplastic resin membranes are carried out by a wet method, a dry method, a pore-forming stretching method, etc., and the wet method is widely used among them. The wet method comprises the steps of adding a solvent to a thermoplastic resin, melt-blending the resultant mixture, and removing the solvent, and the efficiency of this solvent-removing step has influence on the productivity and mechanical properties of the microporous thermoplastic resin membranes. Conventionally used in the solvent-removing step are chlorinated alkanes having low boiling points such as methylene chloride and aliphatic hydrocarbon compounds having low boiling points such as hexane. Though low-boiling-point, chlorinated alkanes are advantageous in high detergency and easy drying, their use is going to be regulated because of environmental pollution. The low-boiling-point aliphatic hydrocarbon compounds are highly likely to ignite and explode during a drying process, and easily evaporated into the air. Therefore, regulations for preventing their discharge are enforced in some prefectures.
On the other hand, for instance, JP 6-256559 A and JP 6-298985 A propose aqueous surfactant solutions, and mixed solvents of organic solvents and water, respectively, as inflammable substitute solvents. However, such aqueous or semi-aqueous solvents do not necessarily have sufficient cleansing power to solvents added to thermoplastic resins.